The invention relates to a fuel cell in the form of plates for producing direct current by electrochemical conversion of hydrogen and fuel gases containing carbon, as well as relating to a portable fuel cell arrangement comprising a stack of such, preferably circular, fuel cells having a high-temperature ceramic electrolyte or a low-temperature polymer electrolyte. The fuel cell arrangement is distinguished by a high power density and space utilization, as well as high efficiency.
Fuel cells having high-temperature ceramic electrolyte as well as those having low-temperature polymer electrolyte are electrochemical appliances, by means of which the chemical energy of a fuel can be converted directly into electrical direct current. Corresponding apparatuses are approaching the stage of readiness for industrial production. Thanks to their efficiency and environmental friendliness, they could very soon partially replace the traditional thermal machines.
Fuel cells having ceramic solid electrolytes have been known for a relatively long time from numerous publications. A large number of arrangements, whose geometries differ from one another, have already been proposed for interconnecting a multiplicity of fuel cells. Designs exist in the form of tubes, rings, corrugations or plates. The latter are based on a filter press, where flat, planar components are arranged in rows to form stacks, and are compressed with one another by means of one or more tie rods.
As a rule, fuel cells are operated with excess fuel. The gas which is not converted has to be burnt downstream from the fuel cell. Elements developed in gas engineering, such as electrical igniters, flame monitors and emergency disconnection, are used for this purpose.
Many options have been investigated for transferring the electrical current from cell to cell. This is no easy task owing to the high operating temperatures in fuel cells with ceramic solid electrolytes, and owing to the oxidizing atmosphere on the oxygen side. For arrangements in the form of plates, the current is carried, as a rule, via electrical connectors at right angles to the plane of the plate within the active space. An alternative connection from cell to cell by means of metallic connecting elements outside the active space in the cell has been proposed, for example, in U.S. Pat. No. 5,338,621.
Some of these configurations in the form of plates are formed from circular individual cells and are disclosed, for example, in EP-A-0 355 420, EP-A-0 437 175, U.S. Pat. No. 5,399,442 and DE 43 33 478, WO-A-86/06762, U.S. Pat. No. 5,445,903. These individual cells are generally supplied with fuel from the inside through at least one channel, which is formed by openings positioned one behind the other. This channel must be sealed as it passes through the layers that carry air, as is evident, for example, from EP-A-0 355 420 or WO-A-86/06762.
In EP-A-0 355 420, WO-A-86/06762 or U.S. Pat. No. 5,399,442 and DE 43 33 478, air, for example, is supplied as the oxidant from the inside by means of at least one channel. In this case, sealing must be provided to prevent air from escaping into the layers carrying the fuel.
An embodiment is disclosed in EP-A-0 437 175, in which reaction air is supplied from the exterior. As a rule, the fuel and air channels in the cell are sealed such that none of these gases can enter the gas space of the other.
The production of the ceramic functional layers comprising the anode, electrolyte and cathode by means of gas flame spraying or plasma spraying onto porous ceramic or metallic substrates is disclosed, for example, in WO-A-86/06762. Furthermore, suitable substrates, one of whose surfaces is coated with ceramic or metallic material, are commercially available. Nickel felt mats, for example, are used for industrial production of fuel cells, coated with nickel powder on one side. The electrolytic layers are then deposited on the surface coated in this way by physical processes (gas flame spraying, plasma spraying, sputtering etc.).
Furthermore, the use of a single tie rod, which is arranged along the cylinder axis, is prior art for a cylindrical arrangement. In arrangements of a related type (for example multilayer filters), centrally arranged tie rods are used. The use of springs between a tie rod and pressure plates to produce a contact pressure which is virtually constant even at different temperatures is also known and is general engineering practice. U.S. Pat. No. 5,514,486 discloses tie rods in fuel cells, which are composed of a single solid material and are used exclusively to compress the cell stack. They are always electrically insulated from all live parts.
For example, U.S. Pat. No 5,514,486 likewise discloses the use of Dow or Nafion membranes as a polymer electrolyte layer. According to this document, one reaction gas is supplied via a central channel and can be diffused towards the periphery of the rotationally symmetrical fuel cell stack, while the other reaction gas enters the fuel cell stack at the periphery, and diffuses towards the centre.
A major problem in a stack of fuel cells is the routing and distribution of the reaction gases. Labyrinths have been proposed, for example in U.S. Pat. No. 5,399,442 and DE 43 33 478 or EP 0 355 420 for influencing the distribution of the gases in an at least approximately rotationally symmetrical stack. These labyrinths are essentially composed of radial apertures and tangential or concentric partially circular channels, which are connected to one another.
On the one hand, the path starting from a region of the fuel cells close to the centre to their periphery is not continuous for the reaction gases: for example, FIG. 1 in EP 0 355 420 or FIG. 8 in U.S. Pat. No. 5,399,442 or DE 43 33 478 shows that, as a result of its radial movement in the region of the radial apertures, a gas strikes the side wallsxe2x80x94which are positioned transversely with respect to the movement at that stagexe2x80x94of the concentric channels and is correspondingly caused to swirl. Once the gas flow has been split into two opposite directions, these flow elements each as strike an adjacent flow element in the region of the next radial aperture. These processes are repeated until the gas has reached the periphery. However, the complicated movement of the gases in no way ensures that they are uniformly distributed. It must therefore be expected that the swirling of the gases when they strike obstructions or meet other flow elements causes a random gas flow which results in a gas distribution that differs greatly from the optimally uniform distribution of the reaction gases. This can lead to local hot spots occurring in the fuel cell or fuel cell stack. As is known, such hot spots have an extremely detrimental effect on correct fuel cell operation. Undesirable pressure losses can also be caused by such changes in the gas flow direction.
On the other hand, the path starting from a region of the fuel cells close to the centre to their periphery is continuous for the reaction gases: for example, FIG. 5 in EP 0 355 420 shows that a gas is intended to carry out a radial movement from a region outside the centre of the fuel cell to its periphery. This eccentric arrangement of the gas inlet leads to a non-homogeneous distribution of reaction gases. Furthermore, in this embodiment, the channels for carrying the gas become much wider towards the periphery whichxe2x80x94owing to the increase in the flow cross sectionxe2x80x94leads to the gas flow slowing down. However, such slowing down is undesirable, since this contributes to a major reduction in the gas reaction. An alternative, continuous gas supply has also been proposed in EP 0 355 420 (FIG. 6), in which the reaction gases move on a spiral from a region close to the centre towards the periphery of the fuel cell. This now actually results in a continuous movement; the length of the path may now, however, have a disadvantageous effect: once again, it results in a severe reduction in the gas reaction, since the concentration of the unused gases reduces, of course, with the length of the path travelled.
A first aspect of the object of the invention is to propose a light and compact fuel cell based on plate elements with a high-temperature ceramic electrolyte or low-temperature polymer electrolyte. According to a second aspect of the object, the intention is to provide a corresponding fuel cell arrangement composed of the fuel cells according to the invention. In this case, the intention is to achieve a more uniform reaction or current distribution within each cell and uniform current transmission between two adjacent cells, at right angles to the plane of the plates, with the gaseous media in the region of the electrodes being supplied and carried away as continuously as possible and in as clear a manner as possiblexe2x80x94while as far as possible avoiding direction changes.
According to the invention, the first aspect of this object is achieved by a fuel cell in the form of discs for axial layering in a stack. The stack is fixed by a tie rod and the fuel cell has an opening to hold the tie rod and to form an inlet for a first gas, and is characterized in that the fuel cell comprises: an ion-conducting electrolyte in the form of a high-temperature ceramic electrolyte or a low-temperature polymer electrolyte; a porous cathode layer as an oxygen electrode or a porous anode layer as a fuel electrode on each side of the electrolyte; a gas-permeable substrate having a first and a second surface and having channels for carrying gases; a small separating plate which is located at least partially on the first surface of the substrate; a large, electrically conductive separating plate which is located on the second surface of the substrate and comprises means for carrying gases in a controlled manner, which gases interact with the channels in the substrate.
According to the invention, the second aspect of this object is achieved in that an arrangement of fuel cells is proposed which is characterized in that the fuel cells are layered axially in a stack, which is fixed by at least one tie rodxe2x80x94which acts between an initial plate and an end platexe2x80x94wherein each tie rod is surrounded by at least one electrically insulating rentring sleeve, such that the first gas can flow through the space between the tie rod and the sleeve, as well as the space between the sleeve and the edge of a channelxe2x80x94which is formed by the stack of fuel cells, and that the second gas can flow through the space which is formed by the apertures (which are placed one above the other in a grid) in the opposite contact regions of the separating plates.
Furthermore, a method is proposed for producing these fuel cells and this arrangement of fuel cells.